The invention relates to a process for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, by indirect hear exchange with the refrigerant mixture of a refrigerant mixture cycle, the refrigerant mixture being compressed in two stages or multiple stages and where the refrigerant mixture is fractionated into at least one lower-boiling refrigerant mixture fraction and into at least one higher-boiling refrigerant mixture fraction.
Currently, most of the baseload-LNG plants are designed as what are termed dual-flow refrigeration processes. In these, the refrigeration energy required for liquefying the hydrocarbon-rich stream or the natural gas is provided by two separate refrigerant mixture cycles which are connected to a refrigerant mixture cycle cascade. A liquefaction process of this type is disclosed, for example, by GB-B 895 094.
In addition, liquefaction processes are known in which the refrigeration energy required for the liquefaction is provided by a refrigerant cycle cascade. but not a refrigerant mixture cycle cascade, see, for example, LINDE Berichte aus Technik und Wissenschaft, issue 75/1997, pages 3-8. The refrigerant cycle cascade described therein consists of a propane or propylene, an ethane or ethylene and a methane refrigeration cycle. Although this refrigerant cycle cascade can be considered to be optimized energetically, it is relatively complicated owing to the nine compressor stages.
In addition, liquefaction processes are known, for example as described in DE-B 19 60 301, in which the refrigeration energy required for the liquefaction is provided by a cascade consisting of a refrigerant mixture cycle and a propane precooling cycle.
Alternatively to the refrigerant or refrigerant mixture cycle cascades mentioned, the refrigeration energy required for the liquefaction can also be provided by only one refrigerant mixture cycle. These what are termed single-flow processes generally require a lower number of apparatuses and machines, compared with the abovementioned cascades, for which reason the capital expenditure costs are lower compared with processes having a plurality of refrigerant (mixture) cycles. In addition, the operation of such single-flow processes is comparatively simple. However, it is a disadvantage that the specific energy requirement for liquefaction is higher compared with processes having a plurality of refrigerant (mixture) cycles.
U.S. Pat. No. 5,535,594 discloses such a single-flow process in which the refrigerant mixture cycle stream is dissolved into two separate refrigerant mixture cycle streams, a higher-boiling refrigerant mixture fraction and a lower-boiling refrigerant mixture fraction, by means of a distillation column which is disposed between the penultimate and final compressor stage of the refrigerant compressor, and by means of a reflux separator which is disposed downstream of the last stage of the refrigerant compressor.
The higher-boiling refrigerant mixture fraction, that is the bottom product of the distillation column, is used for precooling the hydrocarbon-rich stream to be liquefied and the lower-boiling refrigerant fraction and for cooling against itself. The lower-boiling refrigerant mixture fraction, that is the overhead product of the reflux separator, is, after it has been precooled by the higher-boiling refrigerant mixture fraction, used for the liquefaction and subcooling of the hydrocarbon-rich stream to be liquefied and for cooling against itself.
The object of the present invention is to specify a process for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, by means of what is termed a single-flow process, in which the specific energy requirement of the single-flow process is improved with retention of its advantagesxe2x80x94low capital costs and simple and robust operation.
This is achieved according to the invention by means of the fact that
a) the compressed refrigerant mixture is at least partially condensed downstream of the penultimate compressor stage,
b) is fractionated into a higher-boiling liquid fraction and a lower-boiling gas fraction,
c) the lower-boiling gas fraction is compressed to the final pressure,
d) the compressed lower-boiling gas fraction is partially condensed,
e) is fractionated into a lower-boiling gas fraction and a higher-boiling liquid fraction,
f) the higher-boiling liquid fraction is added to the partially condensed refrigerant mixture stream, and
g) the gas fraction forms the lower-boiling refrigerant mixture fraction and the liquid fraction forms the higher-boiling refrigerant mixture fraction of the refrigerant mixture cycle.
In a development of the process according to the invention it is proposed that the higher-boiling liquid fraction is expanded upstream of the addition to the partially condensed refrigerant mixture stream.
An alternative procedure to this described procedure of the invention is characterized in that
a) the compressed refrigerant mixture is partially condensed downstream of each compressor stage and is fractionated each time into a lower-boiling gas fraction and a higher-boiling liquid fraction,
b) only the gas fraction from each partial condensation is further compressed,
c) the liquid fractions, from the second fractionation on, are added to the partially condensed stream from the first compressor stage prior to its fractionation and
d) the gas fraction from the last fractionation forms the lower-boiling refrigerant mixture fraction and the liquid fraction from the first fractionation forms the higher-boiling refrigerant mixture fraction of the refrigerant mixture cycle.
According to an advantageous embodiment of the process of the invention, the liquid fraction produced by means of the fractionation is fed in each case to the preceding pressure stage stream which is to be fractionated upstream of its fractionation.
As a development of the process of the invention it is proposed that the liquid fraction produced by means of the fractionation is expanded upstream of the feed to the preceding pressure stage stream which is to be fractionated.